In recent years, as the wireless communication technologies become matured, a lot of wireless network systems have been derived and put into use in various circumstances. In the public safety network architecture, emergency communication services (e.g., calling the police, the fire station, or the ambulance) or inter-group communication services shall be provided, so there is a need for direct communication between adjacent UEs to make the communication therebetween more efficient. However, the direct communication must be accomplished through specific UEs and a specific frequency band.
Currently, the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) network is experiencing the rapidest development and UEs adopting the 3GPP LTE network are widely used in daily communications. In the 3GPP LTE network, data transmissions between the UEs must be performed by using a base station and a core network as intermediates, so the data transmission must be done within the signal coverage of the base station and via the allocated resources. A UE not located within the signal coverage of the base station is unable to get a resource allocation and, therefore, is unable to communicate with other UEs. Additionally, the current 3GPP LTE network does not provide the service of direct communication between UEs, so the UEs of the current 3GPP LTE network cannot be applied in the public safety network.
Accordingly, an urgent need exists in the art to provide a device to device (D2D) transmission mechanism capable of modifying the current 3GPP LTE network to extend the application thereof to the public safety network so that direct communications between the UEs can be achieved without affecting signal transmissions of the original 3GPP LTE network.